18 



LOWER INVERTEBRATES. 



name to tlie mimmulitie limestone, the tubes have a different arrangement and are 

 A'ery minute ; there is, besides the tubular structure already described, a system of 

 inosculating canals penetrating the septa, which are filled with sarcode during the 

 life of the animal. In all the vitreous forms, each chamber has its own shelly in- 

 vestment, so that the partitions between the chambers are double. Between these 

 walls there is frequently a considerable deposit of calcareous substance, which is 



known as the intermediate skeleton. Through the 

 intermediate skeleton runs the system of canals, 

 which is beautifully shown in ^ozoon, soon to be 

 described ; the canad-system resembling minute 

 Ijranching shrubs. A single species of Ntmunu- 

 lina, Fig. 16, has been described from Florida. 

 Besides the two varieties of shell-structure above 

 described, there is another kind of shell or test very frequently occurring among deep- 

 water species. This is the arenaceous type, in which the shell consists of cemented 

 grains of sand, or of sand and spicules together. 



The nature of the cement which holds together the sand-grains of the arenaceous 

 types is not known ; sometunes the grains are only loosely united, so that the test is 

 more or less flexible, as in Astrorhiza, Fig. 17, a form which is found in Vineyard 

 Sound at depths of only twelve fathoms, but also reaching down to over five hundi-ed 

 fathoms. Some of these have the outside test smoothly plastered by a layer of very 

 fine jiarticles of mud, although composed of irregular large and small grains of sand. 

 No definite aperture, or mouth, has been observed in Astrorhiza, 

 and the sarcode finds its way through the test between the loosely \ s 



cemented grains composing it. In other forms the grains are « ' ; 



very closely cemented, so that some tests will resist the action 

 of warm nitric acid, proving that the cement is neither calcareous 

 or ferruginous. In some cases the sand-grains seem to have a 

 chitinous basis in which they are imbedded. The resemblance 

 between the arenaceous Foraminifera and the porcellanous and 

 vitreous species is striking. Take, for example, Halo2ylirag- 

 mium, and compare it with Glohigei'ina, Fig. 18. 



Indeed, it is true that if we consider only the external foi-ms, 

 WB can find in the three divisions of porcellanous, vitreous, and 



arenaceous forms many species that ai-e so closely related as to be indistinguishable 

 by any specific characteristics. Thus, Cornuspira among the jaorcellanous is the 

 counter-part of Spirillina among the vitreous forms ; and this is distinguishable in 

 form from Ammodiscus among those with sandy tests. 



While some of the tests of the arenaceous group are probably imperforate, others 

 are, without doubt, more or less porous, so that the distinction already made between 

 hyaline and porcellanous forms must also hold good as concerns these. Indeed, cer- 

 tain arenaceous forms have no definite mouth, and the sarcode must find its way 

 through pores in the test. 



The deep-sea investigations that have been carried on of late years have brought to 

 light many new forms belonging to genera which were sujiposed to be very well known. 

 Thus, the shell of Glohigerina, Fig. 18, has been understood, conforming to the descrij)- 

 tion of Dr. Carpenter, to consist of a series of hyaline, ]ierforated, spheroidal chambers 

 arranged in a sjjiral about an axis, each ojjening into a central space in such a manner 



Fig. 17. — Astrorhiza^ en- 

 larged two diameters. 



